1. Field of the Invention
The present invention relates to a duobinary optical transmission apparatus.
2. Description of the Related Art
Typically, a Dense Wavelength Division Multiplexing (DWDM) optical transmission system transmits an optical signal using a single optical fiber in such a way that it enhances transmission efficiency. The optical signal is comprised of a plurality of channels having different wavelengths. In addition, the DWDM optical transmission system has been widely used for a super high-speed Internet network, which has rapidly increasing data transfer quantity, because it transmits optical signals irrespective of a transfer rate. Such a newly developed system accommodates rapidly increasing data traffics as well as transfer requests for high-speed data of more than 40 Gbps.
However, a conventional optical intensity modulation method using a Non-Return to Zero (NRZ) method has a number of limitations. For example, one limitation occurs in increasing the transfer quantity because an abrupt interference and distortion between channels occurs in a prescribed zone less than a channel interval of 50 GHz. Further, DC frequency components of a conventional binary NRZ transmission signal and high-frequency components spreading in a modulation procedure cause nonlinear characteristics and dispersion while they are propagated in an optical fiber medium. Consequently, the transmission distance at a high-speed transfer rate over 10 Gbps is limited.
In recent times, an optical duobinary technique has been intensively researched as a new optical transmission technique for obviating the transmission distance limitation caused by chromatic dispersion. The optical duobinary technique has an advantage in that it reduces the width of the transmission spectrum much more than a general binary transmission method. The transmission distance in a dispersion limitation system is inversely proportional to the square of the transmission spectrum bandwidth. For example, when the transmission spectrum bandwidth is reduced by half, the transmission distance increases by four times.
Also, the carrier wave frequency is suppressed in a duobinary transmission spectrum such that limitations in output optical power caused by the Brillouin Scattering stimulated in an optical fiber are reduced.
FIG. 1 is an exemplary a block diagram of a conventional duobinary optical transmission system.
Referring to FIG. 1, a conventional duobinary optical transmission system includes a precoder 10 for encoding a two-level data signal to a digital signal, Low Pass Filters (LPFs) 20 and 21, drive amplifiers 30 and 31, and a Mach-Zehnder-interferometer-type optical intensity modulator (MZ MOD) 40, and a laser source 50 for generating a carrier wave. The two-level data signal transmitted from the duobinary optical transmission system is applied to a differential precoder 10, and then encoded in the differential precoder 10. The encoded two-level data signal from the precoder 10 is applied to the LPFs 20 and 21, and the LPFs 20 and 21 each convert it into a three-level signal. The three-level signals of the LPFs 20 and 21 are amplified by the drive amplifiers 30 and 31, respectively. The amplified three-level signals function as the driving signal of the MZ MOD 40. The carrier wave generated from the laser source 50 modulates its own optical intensity according to the driving signal of the MZ MOD 40, and is thereby generated as an optical duobinary signal.
FIG. 2 is a block diagram of a precoder 10 of the conventional duobinary optical transmission system shown in FIG. 1.
The conventional precoder includes an XOR gate 11, and a 1-bit delay 12 for delaying an output signal of the XOR gate 11 by just one data bit and feeding back the time-delayed output signal to the XOR gate 11.
However, the aforementioned conventional duobinary optical transmission system has a disadvantage in that the XOR gate, when using a high-speed data signal, unavoidably generates an undesired time delay over at least one data bit. This, in turn causes difficulties in manufacturing such a precoder. Also, the conventional duobinary optical transmission system is affected by Pseudo Random Bit Sequence (PRBS) in producing the three-level data signal using electric LPFs. Thus, generating serious degradation in signal transmission characteristics as a length of the PRBS gradually increases, and thereby causing difficulties in the system implementation process.